Transepithelial vinblastine secretion in two defined MDCK strains displays saturation kinetics; (Strain 1) Km = 2.8 +/- 0.6 microM (six experiments), Vmax 35.9 +/- 1.93 pmol/cm2 per h (six experiments), Strain 2 Km 0.78 +/- 0.36 microM (three experiments), Vmax 12.1 +/- 4.5 pmol/cm2 per h (three experiments). Concentrations of vinblastine > 1 microM are associated with an increased passive vinblastine permeability (PA-B). This correlates with an increased transepithelial conductance/decreased permselectivity, suggesting that this may in part result from increased paracellular conductance. Verapamil inhibits vinblastine secretion, half-maximal inhibition of basal-to-apical flux (JB-A) is observed at 3.4 +/- 0.3 and 1.7 +/- 0.05 microM verapamil for Strain-1 and Strain-2 epithelial layers, respectively. Cellular accumulation of vinblastine across the apical membrane is small with respect to that across the basolateral surfaces. This polarity is unaffected by verapamil. The apical membranes, therefore, possess a low intrinsic permeability to vinblastine. Inhibition of cell growth by vinblastine is enhanced by verapamil. Both the effect of vinblastine, and its enhancement by verapamil, upon cell growth are reduced as initial cell seeding density increases.